Make HTTP middleware easier to use

server/router/handler.go

Index: server/router/handler.go
diff --git a/server/router/handler.go b/server/router/handler.go
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+++ b/server/router/handler.go
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+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package router
+
+// Handler is the type for all request handlers.
+type Handler func(*Context)
+
+// Middleware is a function that accepts a shared context and the next
+// function. Since Middleware is typically part of a chain of functions
+// that handles an HTTP request, it must obey the following rules.
+//
+//     - Middleware must call next if it has not written to the Context
+//       by the end of the function.
+//     - Middleware must not call next if it has written to the Context.
+//     - Middleware must not write to the Context after next is called and
+//       the Context has been written to.
+//     - Middleware may modify the embedded context before calling next.
+type Middleware func(c *Context, next Handler)
+
+// MiddlewareChain is a list of Middleware.
+type MiddlewareChain []Middleware
+
+// run executes the middleware chains m and n and the handler h using
+// c as the initial context. If a middleware in m or n is nil, run
+// simply advances to the next middleware or handler. If h is nil, run
+// panics.
+func run(c *Context, m, n MiddlewareChain, h Handler) {
+	switch {
+	case len(m) > 0:
+		if m[0] != nil {
+			m[0](c, func(ctx *Context) { run(ctx, m[1:], n, h) })
+		} else {
+			run(c, m[1:], n, h)
+		}
+	case len(n) > 0:
+		if n[0] != nil {
+			n[0](c, func(ctx *Context) { run(ctx, nil, n[1:], h) })
+		} else {
+			run(c, nil, n[1:], h)
+		}
+	default:
+		h(c)
+	}
+}